Field of the Invention
The present invention relates to a refrigeration cycle and a method of controlling the same, as well as to an air conditioner. More particularly, the present invention is concerned with a refrigeration cycle which operates with a non-azeotropic mixture refrigerant charged therein, improved to suppress change in the nature of the refrigerant while reducing the amount of refrigerant to be used in the cycle, and also to a method of controlling such a refrigeration cycle and further to an air conditioner incorporating such a refrigeration cycle.
Hitherto, various techniques have been proposed for effecting capacity control of refrigeration cycle by varying the composition ratio of a non-azeotropic mixture refrigerant circulated through the refrigeration cycle.
For instance, Japanese Patent Unexamined Publication No. 61-99066 discloses a heat pump wherein a non-azeotropic mixture refrigerant is introduced into a refrigerant rectifier tower through a three-way valve which is switchable to selectively direct the refrigerant either to the top or the bottom of a rectifier tower, so as to make it possible to largely vary the composition of the refrigerant circulated through the main circuit, whereby the refrigerant composition is controlled continuously to match with the level of the refrigeration load.
Japanese Patent Laid-Open No. 1-58964 discloses a heat pump system in which a gas-liquid separator is connected between an indoor heat exchanger and an outdoor heat exchanger, and a refrigerant tank which is capable of performing heat exchange with a suction gas pipe is connected through a first connecting pipe to an upper part of the gas-liquid separator, the refrigerant tank also being connected to a lower part of the gas-liquid separator through a second connecting pipe having a stop valve, thus forming refrigeration cycle which operates with a non-azeotropic mixture refrigerant. During operation of the refrigeration cycle in cooling mode, gaseous refrigerant rich in low-boiling-point component flowing out from the upper part of the gas-liquid separator is introduced into the refrigerant tank so as to be condensed into liquid phase and is stored therein as liquid refrigerant, whereby a refrigerant rich in high-boiling-point component is circulated through the refrigeration cycle.
Another refrigeration cycle system also has been proposed in which, in order to facilitate maintenance work, the refrigeration cycle is initially charged with refrigerant of an amount corresponding to the internal volume of the maximum length of the connecting piping. In this type of refrigeration cycle, it is necessary to employ a tank to accommodate any surplus refrigerant which is generated when the length of the connecting piping actually used in the operation is short. Conventionally, there are two types of methods for accommodating such surplus refrigerant.
One of these methods employs a liquid receiver as means for accommodating surplus refrigerant, provided at the downstream side of a heat exchanger which serves as a condenser, while the other method, which is disclosed in Japanese Patent Unexamined Publication No. 62-80471, employs an accumulator as means for accommodating surplus refrigerant, provided at the suction portion of the refrigeration cycle.
A description will be given of the refrigeration cycles having means for accommodating surplus refrigerant and charged with non-azeotropic mixture refrigerants. In the refrigeration cycle of the type which employs a liquid receiver, high-pressure refrigerant discharged from the condenser flows into the liquid receiver so as to be stored therein as the surplus refrigerant. The refrigerant flowing into the liquid receiver has a very small degree of quality so that the refrigerant stored in the liquid receiver approximates that of the refrigerant initially charged. Consequently, the composition of the mixture refrigerant circulated through the refrigeration cycle approximates that of the initially charged refrigerant. In contrast, in the refrigeration cycle of the type which employs an accumulator disposed at the suction portion of the refrigeration cycle, refrigerant of a low pressure coming from the evaporator is introduced into the accumulator so as to be accumulated therein as surplus refrigerant. The refrigerant flowing into the accumulator has a very large degree of quality, so that the refrigerant accumulated in the accumulator has a composition which is richer in the high-boiling-point component than the initially charged refrigerant. Consequently, the composition of the mixture refrigerant circulated through the refrigeration cycle is richer in the low-boiling-point component than the composition of the initially charged refrigerant.
In these known methods which employ a mixture refrigerant to enable a change in the composition of the circulated refrigerant or which incorporates means for storing or accumulating surplus refrigerant, no specific consideration is given to adaptability to variation in the length of the piping interconnecting the indoor unit and the outdoor unit nor to protection of global environment.
More specifically, in the known refrigeration system which employs a rectifier tower for varying the composition of the refrigerant circulated through the refrigeration cycle, no surplus refrigerant exists when the length of the piping actually used equals to the maximum design length. In such a case, no fraction of the refrigerant is stored in the refrigerant storage tank and, therefore, it is impossible to vary the composition of the refrigerant circulated through the refrigeration cycle. Conversely, when the refrigerant is stored in the tank to enable control of the composition of the circulated refrigerant, the effective amount of the refrigerant circulated through the refrigeration cycle becomes insufficient, with the result that the efficiency of the refrigeration cycle is reduced. When the amount of the initial charge of the refrigerant is increased to optimize the effective amount of refrigerant circulated through the refrigeration cycle, the amount of refrigerant leaking from the refrigeration cycle or freed when the refrigeration cycle is disposed is increased to accelerate the warming of the air on the earth.
The known refrigeration cycle of the type employing a gas-liquid separator to enable control of the composition of the circulated refrigerant makes it possible to enrich the refrigerant in high-boiling-point component during cooling operation. In the operation in heating mode, however, the liquid refrigerant in the refrigerant tank evaporates to flow into the gas-liquid separator, so that the composition of the circulated refrigerant is rendered rich in low-boiling-point component. Thus, the composition of the circulated refrigerant is changed according to the mode of the operation. This poses problems when the compressor is driven by a constant-speed motor, such as a large difference in the power between the heating and cooling operations, or rise of the refrigerant pressure to a level exceeding the maximum allowable pressure in the refrigeration cycle.
The known refrigeration cycle employing an accumulator as means for accumulating surplus refrigerant has suffered from the following disadvantage, since this type of refrigeration cycle has not been designed to use a non-azeotropic mixture refrigerant.
Namely, a liquid receiver is essentially required to accommodate a change in the rate of circulation of the refrigerant which varies according to the thermal load during the operation of the refrigeration cycle in the cooling or heating mode. Meanwhile, non-azeotropic mixture refrigerant exhibits different compositions depending on whether it is in liquid phase or gaseous phase, as shown in FIG. 12. In the refrigeration cycle in which the liquid receiver is connected between the heat exchanger serving as an evaporator and the compressor of the cycle, when the refrigerant flowing into the liquid receiver has a large degree of quality (composition A in FIG. 12), refrigerant of a composition (composition B in FIG. 12) rich in HFC-134a, which is a high-boiling-point component of the refrigerant, is stored in the liquid receiver. Therefore, in steady operation of the refrigeration cycle, a refrigerant rich in HFC-32 is circulated through the refrigeration cycle. Thus, the composition of the refrigerant circulated through the refrigeration cycle differs from that of the initially charged refrigerant. HFC-32 is the low-boiling-point component so that enrichment in this component causes a rise in the operation pressure of the refrigeration cycle, causing the pressure at the high-pressure side to exceed the maximum allowable pressure of the refrigeration cycle. The increased pressure also enhances the tendency of leak of the refrigerant. Leakage of HFC-32 is dangerous because this component is inflammable.
In some cases, component or components such as a liquid receiver are beforehand charged with the refrigerant and then connected. The liquid receiver is required to accommodate any surplus refrigerant also in these cases, when the length of the piping actually used is small. Consequently, the same problems as those stated above have been encountered.